DESCRIPTION: (Applicant's Description) Several cancers are known to arise from genetic defects in DNA repair genes. We propose to test whether concurrent germline defects in base-excision DNA repair genes are associated with sporadic breast cancer. Our focus is on small genetic variations we call eSNPs, exon Single Nucleotide Polymorphisms, in the coding region and the flanking control regions in the exons of genes. Specific Aim 1 is to screen the germline DNA of hundreds of women, half with sporadic breast cancer, for the presence of eSNPs in base-excision repair genes, and to test for the association of these eSNPs with breast cancer. In Specific Aim 2, we propose to develop a novel methodology that can be used to rapidly screen for eSNPs in these genes in thousands of individuals. In base-excision repair, more than one enzyme is specific for a given DNA damage. We hypothesize that when deleterious germline eSNPs occur concurrently in different genes of base-excision repair enzymes, the repair efficiency would decrease and somatic mutation rates would rise, and lead to cancer. Breast cancer that originates from multiple germline defects may seem to be sporadic. Currently it is impractical to screen for eSNPs in tens of repair genes in thousands of individuals. Initially, we will focus on the T/G mismatch that arises from spontaneous deamination of 5-methyl-cytosine residues in the DNA. Seven base-excision repair genes that correct this mismatch will be screened for eSNPs. The genes are: TDG, MEDI (MBD4), APE, NTHLI, PCNA, FEN1, and POL . The eSNP detection method we will use is based on a mismatch-specific CEL I nuclease we discovered. We propose to use a new mismatch-specific property of CEL I to perform affinity selection of total eSNPs, and also to enable affinity subtraction to remove the common alleles of eSNPs. The end product will be a highly emiched eSNP collection specific for sporadic breast cancer. Our proposed procedure will lead to a complete database of common and rare eSNPs in these genes so that existing technologies can do mass genotyping studies.